Before she slipped back into a bikini for Saturday's party at Las Vegas' Wet Republic -- the last time she was photographed in a two-piece was April 2010 -- Heidi Montag spent some serious time in the gym.

"I've been working out from, like, 5 a.m. to 7 p.m. for two months now. I've been working out really hard because I had this pool party and I was like, I have to be in shape," she told Us Weekly. "And I was actually a lot overweight. It was the most I've ever been because I've kind of been in hiding eating pie with my husband and puppies, so I needed to get back in shape."

At her heaviest, the 5'2" reality star claimed she weighed 130 pounds; she's currently back down to 103. How did the "Hills" alum lose 27 pounds so quickly?

"I've been running a lot, and I've been doing weights," she said. "When you work out, you boost your metabolism, so you have to [make sure you eat enough]."

Montag -- who listed pie and fried chicken as her weaknesses -- said she's been noshing on apples, cherries and salads to slim down. After famously undergoing 10 cosmetic surgeries in late 2009, Montag added that decided to take a year off from the gym.

"My breasts, because they're so big, really needed some time," she explained. "So I'm just starting to work out again after my surgery. Sometimes I get shooting pains, but I hear that's normal."